Some form of plating is often used in the processing of semiconductor wafers to deposit multiple layers of conductive metals on a semiconductor wafer for forming electrically conductive regions such as, for example, metal bumps for bonding. Multiple wafers are often processed at one time during the plating processing of semiconductor wafers. It is therefore desirable that the plating apparatus used plates wafers quickly and efficiently.
Plating involves the deposition of an adherent metallic layer onto a conductive object, such as a semiconductor wafer, by placing the wafer into an electrolytic bath composed of a salt solution consisting of the metal to be plated onto the wafer. Two plating techniques are known. Electroplating is a plating technique which passes a DC current through the solution to affect the transfer of metal ions onto the cathodic surface of the conductive object. Another plating technique is known as electroless plating which proceeds by an exchange of reaction between the metal complexes in a solution and the particular metal to be plated without requiring an externally applied electric current.
FIGS. 1-3 illustrate two prior art plating tanks. FIG. 1 shows an example of a rack plating tank 10 having one or more fluid nozzles 14 for fluid agitation located near the bottom of tank 10 between an anode 12 and a rack 16. FIG. 2 shows an example of rack 16, which supports one or more semiconductor wafers 18 of the same or different sizes on one or more surface 22 of rack 16. Not shown are probes which carry electric current to cathodes on the wafer 18 through an opening in the photoresist on the face of wafer. Salt solution 24 is forced over the wafers to provide agitation between the wafer and the solution. Rack 16 can be mechanically moved in salt solution 24 to increase the aggregation of the salt solution to the cathodic surfaces on the wafers. Rack plating tank 10 has a simple set-up which accommodates easily different sizes and quantities of wafers to be electroplated. However, spacing the anodes 12 away from wafers 18 to allow for fluid nozzles 14 reduces the anode efficiency of metal transfers and thus also reduces the efficiency of plating tank 10.
FIG. 3 is an example of a cross-sectional view of a prior art fountain plating tank 30 implementing one or more plating stations 32 with each plating station accommodating a wafer 18 placed perpendicular to a flow of a salt solution. Each plating station 32 has a round vertical tube 34 slightly wider than the diameter of wafer 18, with an anode placed close to and parallel to wafer surface. Wafer 18 is supported between the top edges of tube 34 with contact tips 38. Gap 39 is formed between the wafer edge and the top edge of tube 34. One or more contact tips 38 are placed between the wafer edge and the top edge of tube 34, to bridge gap 39 while allowing a flow 14 of salt solution 24 between wafer 18 and the top edge of tube 34. Gap 39 is critical for providing a proper flow rate 14 of the salt solution. This gap has to be duplicated accurately on all plating stations 32 to obtain identical flow rates between all plating stations in the system. Each wafer 18 must also be placed parallel to a fluid level 36 of salt solution 24 contained within tube 34. The walls of tube 34 must be perpendicular to wafer 18 while parallel to each other to ensure that fluid flow 14 is perpendicular to wafer 18. To electroplate wafer 18, an electrical contact is made through contact tips 38 to cathodes on wafer 18 as solution 24 is pumped up tube 34 to flow over wafer 18. This provides very efficient contact of the solution with the cathodic metal on the contact tips and wafers.
Fountain plating tank 30 provides high agitation efficiency between the solution and each wafer. However, to provide proper fluid flow, each plating station set-up in tank 30 must be precisely duplicated throughout the fountain plating system to ensure identical flow rate to each station. This requires that all stations in the plating tank to contain a wafer 18 even if the wafer need not be electroplated. The fountain plating tank 30 is therefore inefficient for reasons that it requires one wafer per station, and that it is difficult to modify for electroplating wafers of different sizes without changing each wafer station in the tank.
A more efficient plating tank is needed that combines the aggregation efficiency of the fountain plater with the flexibility in processing different quantities and sizes of wafers of the rack platers.